This application is based on application No. 99-22764, filed in the Korean Industrial Property Office on Jun. 17, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to an active material for a lithium secondary battery and a method of preparing the same, more particularly, to an active material for a lithium secondary battery in which an oxygen (O) in a LiaNi1xe2x88x92xxe2x88x92yCoxMyO2 is substituted with F or S, and a method of preparing the same.
(b) Description of the Related Art
Due to technological advances in appliance miniaturization and weight reduction, and increased functionality of cordless portable appliances such as video cameras, personal phones, and personal computers, there are escalating requirements for the sources of electric power used to driving these appliances. Particularly, there have been advances in developing and studying rechargeable lithium secondary batteries around the world, anticipating the need for a battery with a high energy density.
A lithium secondary battery uses as an anode and a cathode materials which can intercalate and deintercalate lithium ions, and is prepared by filling organic or polymer electrolyte between the cathode and the anode to move the lithium ions. The battery generates electric energy by a redox reaction when lithium ions intercalate and deintercalate in the cathode and in the anode.
Lithium secondary batteries use carbon materials or lithium metals as anodes and intercalatable/deintercalatable chalcogenide compounds as cathodes. Carbon materials are substituted for lithium metals because the latter, when used as an anode, has the disadvantage of educing dendrites with the associated danger of explosion and a reduction in recharging efficiency.
On the other hand, complex metal oxides such as LiCoO2, LiMn2O4, LiNi1xe2x88x92xCoxO2 (0 less than X less than 1), and LiMnO2 are now being studied for a cathode use because chrome oxide, MnO2, etc. that were initially used have problems with low recharge efficiency and safety.
Manganic positive active materials such as LiMn2O4, LiMnO2, etc, or cobaltic positive active materials such as LiCoO2, etc, had been developed, but they have the limits of capacity of 120 mAh/g and 160 mAh/g respectively when recharged at 4.3 V. Also, LiCoO2 has been widely used due to having the high voltage capacity, excellent electrode properties, and an electro-conductivity of 10xe2x88x922 to 1 S/cm at ambient temperature, but it has low stability when recharged and discharged at a high current rate.
There have been developments in the study of nickelic positive active materials that show a discharge capacity more than 20% greater than cobaltic positive active materials.
Lithium secondary batteries using nickelic positive active materials have the potential to make high capacity batteries due to their high discharge capacity, but more development of nickelic active materials is needed in order to overcome defects associated with their low durability and the structural instability of LiNi1xe2x88x92xCoxO2 (0 less than x less than 1).
Synthesizing methods employing solid state processes, co-precipitation methods, polymer chelating agents, etc, have been developed and researched thus far on LiNi1xe2x88x92xMxO2 (0 less than x less than 1) powder with some Ni substituted with Co, Mn, etc, for improving structural safety features, discharge capacities, and life span properties of the basic nickel based cathode compound, LiNiO2.
LiNiO2 has disadvantages in that it is difficult to synthesize, is not practical to use in a battery because of poor durability, and its capacity decreases suddenly during continuous discharge-recharge cycles due to instabilities caused by its repeated structural change from monoclinic to hexagonal and back, in spite of having a discharge capacity of 200 mAh/g at 1.0 C.
To solve these problems, Co is added to LiNiO2 in order to stabilize the structure, but this causes the problem that the capacity of LiNiO2 decreases relative to the amount of Co added, and this quantity must be more than 30 mole %.
To improve the structural stability, LiNi1xe2x88x92xMxO2 (M is a metal such as Co or Mn, etc, 0 less than x less than 1) and LiNi1xe2x88x92xCoxMyO2 (M is a metal such as Al, Mg, Sr, La, Ce, etc, 0 less than x less than 1, 0.01 less than y less than 0.1) were developed. However, these nikelic positive active materials also have defects of structural instabilities, and this defect causes the stability of the system of lithium secondary battery to decrease.
It is an object of the present invention to provide a positive active material for a Li secondary battery, wherein LiaNi1xe2x88x92xxe2x88x92yCoxMyO2xe2x88x92zFz and LiaNi1xe2x88x92xxe2x88x92yCoxMyO2xe2x88x92zSz (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0xe2x89xa6x less than 0.99, 0.01xe2x89xa6yxe2x89xa60.1, 0.01xe2x89xa6zxe2x89xa60.1, and 1.00xe2x89xa6axe2x89xa61.1) powders in which an oxygen (O) in LiaNi1xe2x88x92xxe2x88x92yCoxMyO2 is substituted with F or S are synthesized to improve the durability, capacity, and structural stability of the battery.
It is another object to provide a method of preparation of the positive active material for a Li secondary battery.
In order to achieve these other objects, the present invention provides positive active materials for Li secondary battery in which an oxygen (O) in LiaNi1xe2x88x92xxe2x88x92yCoxMyO2 (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0xe2x89xa6x less than 0.99, 0.01xe2x89xa6yxe2x89xa60.1, 0.01xe2x89xa6zxe2x89xa60.1, and 1.00xe2x89xa6axe2x89xa61.1) is substituted with F or S, that is, positive active materials selected from the group consisting of the following formulae 1 and 2:
LiaNi1xe2x88x92xxe2x88x92yCoxMyO2xe2x88x92zFzxe2x80x83xe2x80x83[Formula 1],
LiaNi1xe2x88x92xxe2x88x92yCoxMyO2xe2x88x92zSzxe2x80x83xe2x80x83[Formula 2],
and
where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti, and wherein 0xe2x89xa6x less than 0.99, 0.01xe2x89xa6yxe2x89xa60.1, 0.01xe2x89xa6zxe2x89xa60.1, and 1.00xe2x89xa6axe2x89xa61.1.
Also, the present invention further provides a method of preparation the positive active material selected from the group consisting of the formulae 1 and 2.
The method comprises a step of synthesizing Ni1xe2x88x92xxe2x88x92yCoxMy(OH)2 by a coprecipitation method; a step of mixing the material with LiOH, and LiF or NaS powder; and a step of producing the positive active compound of the formulae 1 and 2 by heating and cooling the mixture.